Numerous Superfund sites around the United States are contaminated with carcinogenic and suspected carcinogenic hazardous compounds. For many communities these sites threaten existing or potential sources of drinking water and surface waters. One technology for removing and controlling these hazards associated with aquifers is in situ bioremediation. We propose a project addressing a bioremediation of chlorinated aliphatic contaminants at hazardous waste sites. The study, an extension of present work by our multi-investigator team, will investigate key issues in understanding and applying bioremediation in contaminated aquifers, Basic microbial processes of reductive dechlorination of chloroethenes and anaerobic and aerobic oxidation of less chlorinated ethenes are not understood sufficiently to accurately predict the extent and rate of these processes at chlorinated ethenes are not understood sufficiently to accurately predict the extent and rate of these processes at specific sites. We propose studies to understand when the various bioremediation activities can occur and to find methods to measure their presence and rates. We will examine the anaerobic reductive transformations of perchloroethylene (PCE), trichloroethylene (TCE) and the anaerobic and anaerobic oxidation of TCE, cis-1,2- dichloroethene (cDCE) and vinyl chloride (VC). The studies include applications of molecular techniques for characterization of enrichments and of aquifer microbial communities, and they include laboratory studies to develop basic understanding of the microbial processes, their rates and their possible biostimulation. Prediction of bioremediation needs further development of modeling tools, incorporating experimental results on biodegradation mechanisms and rates, in order to predict natural attenuation. and biostimulation at contaminated sites. Our effort will be conducted using a contaminated field site as the source of material used in our laboratory experiments and as the site for the development and testing of our methods and models.